The present invention relates to the field of hydraulics. More particularly, this invention relates to a hydrostatic loop dump valve for reducing the input torque required by a hydrostatic unit, especially during cold oil starting.
It is well known to utilize closed circuit or closed loop hydrostatic transmissions for propulsion of tractors and other vehicles. Conventionally, a variable hydrostatic transmission or unit is directly driven by the engine of the tractor. In cold weather engine starting conditions, the torque required to turn the hydrostatic unit, especially if it is in stroke, is much greater than under warmer conditions. The additional torque load from the hydrostatic unit can overload the engine starting system, making cold weather starting difficult or impossible.
If the hydrostatic variable unit is set at zero displacement (neutral), starting torque will be low. However, the vehicle original equipment manufacturer cannot guarantee that the neutral condition can be achieved before starting the engine. A cost effective method is needed to reduce the hydrostatic torque during starting, especially under cold weather conditions.
Therefore, a primary objective of the present invention is the provision of a hydrostatic loop dump valve for reducing the hydrostatic torque during starting.
Another objective of the present invention is the provision of a hydrostatic unit with one or more loop dump valves that reduce the hydrostatic unit input torque under cold oil conditions, even when the unit is not started in the neutral position.
Another objective of the present invention is the provision of a hydrostatic loop dump valve that automatically closes when there is a build up of charge pressure in either side of the loop so that the hydrostatic unit will not be damaged by running at higher speeds without oil.
Another objective of the present invention is the provision of a hydrostatic loop dump valve which automatically drains the loop when the engine is shut off.
Another objective of the present invention is the provision of a hydrostatic loop dump valve and hydrostatic transmission that is relatively inexpensive to manufacture, simple, and durable and reliable in use.
These and other objectives will be apparent from the drawings as well as from the description and claims which follow.
The present invention relates to the field of hydraulics. More particularly, this invention relates to a hydrostatic loop dump valve for reducing the input torque required by a hydrostatic unit during cold oil starting.
The hydrostatic loop dump valve of the present invention includes a normally biased open dump valve connecting one or more of the system pressure lines to the reservoir. The dump valve(s) will open and drain oil from the loop by gravity when the engine is not running. When the engine has started and turns the hydrostatic unit at sufficient speeds to generate significant charge pressure, the dump valves close and the unit operates normally. Thus, the initial input torque required to start the engine and the hydrostatic unit is minimized without damaging the hydrostatic unit or interfering with its normal operation after startup.
In one embodiment, a simple dump valve is provided for each side of the loop. The dump valve includes a check ball normally spring biased out of selective engagement with a seat and operatively connected to the sump or case drain. These dump valves are placed at the lowest portion of the loop so that the fluid drains out of the loop when the check ball is unseated. In another similar embodiment, the dump valve is a velocity fuse.
In another embodiment of this invention, a novel loop flushing shuttle valve and dump valve combination is utilized to insure that the dump valve will operate to dump fluid from the system pressure lines to the reservoir, regardless of whether the hydrostatic unit is in neutral, forward or reverse.